Method retainAll

guava/src/com/google/common/collect/Maps.java:4126–4139

(Collection<?> c)

    }

    @Override
    public boolean retainAll(Collection<?> c) {
      try {
        return super.retainAll(checkNotNull(c));
      } catch (UnsupportedOperationException e) {
        Set<K> toRetain = new HashSet<>();
        for (Entry<K, V> entry : map().entrySet()) {
          if (c.contains(entry.getValue())) {
            toRetain.add(entry.getKey());
          }
        }
        return map().keySet().retainAll(toRetain);
      }
    }

    @Override
    public int size() {